Add MonotonicallyIncreasingTime to V8 Platform.

Adds a MonotonicallyIncreasingTime() function to the Platform API to enable the
embedder to provide a time API to V8 which will be used for IdleNotification deadlines.

BUG=417668
LOG=Y
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/632663002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24411 ce2b1a6d-e550-0410-aec6-3dcde31c8c00